RFC 3779 X.509 Extensions for IP Addr and AS ID June 20041. Introduction
This document defines two X.509 v3 certificate extensions that
authorize the transfer of the right-to-use for a set of IP addresses
and autonomous system identifiers from IANA through the regional
Internet registries (RIRs) to Internet service providers (ISPs) and
user organizations. The first binds a list of IP address blocks,
often represented as IP address prefixes, to the subject (private key
holder) of a certificate. The second binds a list of autonomous
system (AS) identifiers to the subject (private key holder) of a
certificate. The issuer of the certificate is an entity (e.g., the
IANA, a regional Internet registry, or an ISP) that has the authority
to transfer custodianship of ("allocate") the set of IP address
blocks and AS identifiers to the subject of the certificate. These
certificates provide a scalable means of verifying the right-to-use
for a set of IP address prefixes and AS identifiers. They may be
used by routing protocols, such as Secure BGP [S-BGP], to verify
legitimacy and correctness of routing information, or by Internet
routing registries to verify data that they receive.
Sections 2 and 3 specify several rules about the encoding of the
extensions defined in this specification that MUST be followed.
These encoding rules serve the following purposes. First, they
result in a unique encoding of the extension's value; two instances
of an extension can be compared for equality octet-by-octet. Second,
they achieve the minimal size encoding of the information. Third,
they allow relying parties to use one-pass algorithms when performing
certification path validation; in particular, the relying parties do
not need to sort the information, or to implement extra code in the
subset checking algorithms to handle several boundary cases
(adjacent, overlapping, or subsumed ranges).
1.1. Terminology
It is assumed that the reader is familiar with the terms and concepts
described in "Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile" [RFC3280], "INTERNET
PROTOCOL" [RFC791], "Internet Protocol Version 6 (IPv6) Addressing
Architecture" [RFC3513], "INTERNET REGISTRY IP ALLOCATION GUIDELINES"
[RFC2050], and related regional Internet registry address management
policy documents. Some relevant terms include:
allocate - the transfer of custodianship of a resource to an
intermediate organization (see [RFC2050]).
assign - the transfer of custodianship of a resource to an end
organization (see [RFC2050]).
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RFC 3779 X.509 Extensions for IP Addr and AS ID June 2004
Autonomous System (AS) - a set of routers under a single technical
administration with a uniform policy, using one or more interior
gateway protocols and metrics to determine how to route packets
within the autonomous system, and using an exterior gateway
protocol to determine how to route packets to other autonomous
systems.
Autonomous System number - a 32-bit number that identifies an
autonomous system.
delegate - transfer of custodianship (that is, the right-to-use) of
an IP address block or AS identifier through issuance of a
certificate to an entity.
initial octet - the first octet in the value of a DER encoded BIT
STRING [X.690].
IP v4 address - a 32-bit identifier written as four decimal numbers,
each in the range 0 to 255, separated by a ".". 10.5.0.5 is an
example of an IPv4 address.
IP v6 address - a 128-bit identifier written as eight hexadecimal
quantities, each in the range 0 to ffff, separated by a ":".
2001:0:200:3:0:0:0:1 is an example of an IPv6 address. One string
of :0: fields may be replaced by "::", thus 2001:0:200:3::1
represents the same address as the immediately preceding example.
(See [RFC3513]).
prefix - a bit string that consists of some number of initial bits of
an address, written as an address followed by a "/", and the
number of initial bits. 10.5.0.0/16 and 2001:0:200:3:0:0:0:0/64
(or 2001:0:200:3::/64) are examples of prefixes. A prefix is
often abbreviated by omitting the less-significant zero fields,
but there should be enough fields to contain the indicated number
of initial bits. 10.5/16 and 2001:0:200:3/64 are examples of
abbreviated prefixes.
Regional Internet Registry (RIR) - any of the bodies recognized by
IANA as the regional authorities for management of IP addresses
and AS identifiers. At the time of writing, these include
AfriNIC, APNIC, ARIN, LACNIC, and RIPE NCC.
right-to-use - for an IP address prefix, being authorized to specify
the AS that may originate advertisements of the prefix throughout
the Internet. For an autonomous system identifier, being
authorized to operate a network(s) that identifies itself to other
network operators using that autonomous system identifier.
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RFC 3779 X.509 Extensions for IP Addr and AS ID June 2004
subsequent octets - the second through last octets in the value of a
DER encoded BIT STRING [X.690].
trust anchor - a certificate that is to be trusted when performing
certification path validation (see [RFC3280]).
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, and MAY, and OPTIONAL, when they appear in
this document, are to be interpreted as described in [RFC2119].
2. IP Address Delegation Extension
This extension conveys the allocation of IP addresses to an entity by
binding those addresses to a public key belonging to the entity.
2.1. Context
IP address space is currently managed by a hierarchy nominally rooted
at IANA, but managed by the RIRs. IANA allocates IP address space to
the RIRs, who in turn allocate IP address space to Internet service
providers (ISPs), who may allocate IP address space to down stream
providers, customers, etc. The RIRs also may assign IP address space
to organizations who are end entities, i.e., organizations who will
not be reassigning any of their space to other organizations. (See
[RFC2050] and related RIR policy documents for the guidelines on the
allocation and assignment process).
The IP address delegation extension is intended to enable
verification of the proper delegation of IP address blocks, i.e., of
the authorization of an entity to use or sub-allocate IP address
space. Accordingly, it makes sense to take advantage of the inherent
authoritativeness of the existing administrative framework for
allocating IP address space. As described in Section 1 above, this
will be achieved by issuing certificates carrying the extension
described in this section. An example of one use of the information
in this extension is an entity using it to verify the authorization
of an organization to originate a BGP UPDATE advertising a path to a
particular IP address block; see, e.g., [RFC1771], [S-BGP].
2.1.1. Encoding of an IP Address or Prefix
There are two families of IP addresses: IPv4 and IPv6.
An IPv4 address is a 32-bit quantity that is written as four decimal
numbers, each in the range 0 through 255, separated by a dot (".").
10.5.0.5 is an example of an IPv4 address.
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RFC 3779 X.509 Extensions for IP Addr and AS ID June 2004
An IPv6 address is a 128-bit quantity that is written as eight
hexadecimal numbers, each in the range 0 through ffff, separated by a
semicolon (":"); 2001:0:200:3:0:0:0:1 is an example of an IPv6
address. IPv6 addresses frequently have adjacent fields whose value
is 0. One such group of 0 fields may be abbreviated by two
semicolons ("::"). The previous example may thus be represented by
2001:0:200:3::1.
An address prefix is a set of 2^k continuous addresses whose most-
significant bits are identical. For example, the set of 512 IPv4
addresses from 10.5.0.0 through 10.5.1.255 all have the same 23
most-significant bits. The set of addresses is written by appending
a slash ("/") and the number of constant bits to the lowest address
in the set. The prefix for the example set is 10.5.0.0/23, and
contains 2^(32-23) = 2^9 addresses. The set of IPv6 addresses
2001:0:200:0:0:0:0:0 through 2001:0:3ff:ffff:ffff:ffff:ffff:ffff
(2^89 addresses) is represented by 2001:0:200:0:0:0:0:0/39 or
equivalently 2001:0:200::/39. A prefix may be abbreviated by
omitting the least-significant zero fields, but there should be
enough fields to contain the indicated number of constant bits. The
abbreviated forms of the example IPv4 prefix is 10.5.0/23, and of the
example IPv6 prefix is 2001:0:200/39.
An IP address or prefix is encoded in the IP address delegation
extension as a DER-encoded ASN.1 BIT STRING containing the constant
most-significant bits. Recall [X.690] that the DER encoding of a BIT
STRING consists of the BIT STRING type (0x03), followed by (an
encoding of) the number of value octets, followed by the value. The
value consists of an "initial octet" that specifies the number of
unused bits in the last value octet, followed by the "subsequent
octets" that contain the octets of the bit string. (For IP
addresses, the encoding of the length will be just the length.)
In the case of a single address, all the bits are constant, so the
bit string for an IPv4 address contains 32 bits. The subsequent
octets in the DER-encoding of the address 10.5.0.4 are 0x0a 0x05 0x00
0x04. Since all the bits in the last octet are used, the initial
octet is 0x00. The octets in the DER-encoded BIT STRING is thus:
Type Len Unused Bits ...
0x03 0x05 0x00 0x0a 0x05 0x00 0x04
Similarly, the DER-encoding of the prefix 10.5.0/23 is:
Type Len Unused Bits ...
0x03 0x04 0x01 0x0a 0x05 0x00
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RFC 3779 X.509 Extensions for IP Addr and AS ID June 2004
In this case, the three subsequent octets contain 24 bits, but the
prefix only uses 23, so there is one unused bit in the last octet,
thus the initial octet is 1 (the DER require that all unused bits
MUST be set to zero-bits).
The DER-encoding of the IPv6 address 2001:0:200:3:0:0:0:1 is:
Type Len Unused Bits ...
0x03 0x11 0x00 0x20 0x01 0x00 0x00 0x02 0x00 0x00 0x03
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x01
and the DER-encoding of the prefix 2001:0:200/39, which has one
unused bit in the last octet, is:
Type Len Unused Bits ...
0x03 0x06 0x01 0x20 0x01 0x00 0x00 0x02
2.1.2. Encoding of a Range of IP Addresses
While any contiguous range of IP addresses can be represented by a
set of contiguous prefixes, a more concise representation is achieved
by encoding the range as a SEQUENCE containing the lowest address and
the highest address, where each address is encoded as a BIT STRING.
Within the SEQUENCE, the bit string representing the lowest address
in the range is formed by removing all the least-significant zero-
bits from the address, and the bit string representing the highest
address in the range is formed by removing all the least-significant
one-bits. The DER BIT STRING encoding requires that all the unused
bits in the last octet MUST be set to zero-bits. Note that a prefix
can always be expressed as a range, but a range cannot always be
expressed as a prefix.
The range of addresses represented by the prefix 10.5.0/23 is
10.5.0.0 through 10.5.1.255. The lowest address ends in sixteen
zero-bits that are removed. The DER-encoding of the resulting
sixteen-bit string is:
Type Len Unused Bits ...
0x03 0x03 0x00 0x0a 0x05
The highest address ends in nine one-bits that are removed. The DER-
encoding of the resulting twenty-three-bit string is:
Type Len Unused Bits ...
0x03 0x04 0x01 0x0a 0x05 0x00
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RFC 3779 X.509 Extensions for IP Addr and AS ID June 20042.2.2. Criticality
This extension SHOULD be CRITICAL. The intended use of this
extension is to connote a right-to-use for the block(s) of IP
addresses identified in the extension. A CA marks the extension as
CRITICAL to convey the notion that a relying party MUST understand
the semantics of the extension to make use of the certificate for the
purpose it was issued. Newly created applications that use
certificates containing this extension are expected to recognize the
extension.
2.2.3. Syntax
id-pe-ipAddrBlocks OBJECT IDENTIFIER ::= { id-pe 7 }
IPAddrBlocks ::= SEQUENCE OF IPAddressFamily
IPAddressFamily ::= SEQUENCE { -- AFI & optional SAFI --
addressFamily OCTET STRING (SIZE (2..3)),
ipAddressChoice IPAddressChoice }
IPAddressChoice ::= CHOICE {
inherit NULL, -- inherit from issuer --
addressesOrRanges SEQUENCE OF IPAddressOrRange }
IPAddressOrRange ::= CHOICE {
addressPrefix IPAddress,
addressRange IPAddressRange }
IPAddressRange ::= SEQUENCE {
min IPAddress,
max IPAddress }
IPAddress ::= BIT STRING
2.2.3.1. Type IPAddrBlocks
The IPAddrBlocks type is a SEQUENCE OF IPAddressFamily types.
2.2.3.2. Type IPAddressFamily
The IPAddressFamily type is a SEQUENCE containing an addressFamily
and ipAddressChoice element.
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RFC 3779 X.509 Extensions for IP Addr and AS ID June 20042.2.3.3. Element addressFamily
The addressFamily element is an OCTET STRING containing a two-octet
Address Family Identifier (AFI), in network byte order, optionally
followed by a one-octet Subsequent Address Family Identifier (SAFI).
AFIs and SAFIs are specified in [IANA-AFI] and [IANA-SAFI],
respectively.
If no authorization is being granted for a particular AFI and
optional SAFI, then there MUST NOT be an IPAddressFamily member for
that AFI/SAFI in the IPAddrBlocks SEQUENCE.
There MUST be only one IPAddressFamily SEQUENCE per unique
combination of AFI and SAFI. Each SEQUENCE MUST be ordered by
ascending addressFamily values (treating the octets as unsigned
quantities). An addressFamily without a SAFI MUST precede one that
contains an SAFI. When both IPv4 and IPv6 addresses are specified,
the IPv4 addresses MUST precede the IPv6 addresses (since the IPv4
AFI of 0001 is less than the IPv6 AFI of 0002).
2.2.3.4. Element ipAddressChoice and Type IPAddressChoice
The ipAddressChoice element is of type IPAddressChoice. The
IPAddressChoice type is a CHOICE of either an inherit or
addressesOrRanges element.
2.2.3.5. Element inherit
If the IPAddressChoice CHOICE contains the inherit element, then the
set of authorized IP addresses for the specified AFI and optional
SAFI is taken from the issuer's certificate, or from the issuer's
issuer's certificate, recursively, until a certificate containing an
IPAddressChoice containing an addressesOrRanges element is located.
2.2.3.6. Element addressesOrRanges
The addressesOrRanges element is a SEQUENCE OF IPAddressOrRange
types. The addressPrefix and addressRange elements MUST be sorted
using the binary representation of:
<lowest IP address in range> | <prefix length>
where "|" represents concatenation. Note that the octets in this
representation (a.b.c.d | length for IPv4 or s:t:u:v:w:x:y:z | length
for IPv6) are not the octets that are in the DER-encoded BIT STRING
value. For example, given two addressPrefix:
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RFC 3779 X.509 Extensions for IP Addr and AS ID June 2004
IP addr | length DER encoding
---------------- ------------------------
Type Len Unused Bits...
10.32.0.0 | 12 03 03 04 0a 20
10.64.0.0 | 16 03 03 00 0a 40
the prefix 10.32.0.0/12 MUST come before the prefix 10.64.0.0/16
since 32 is less than 64; whereas if one were to sort by the DER BIT
STRINGs, the order would be reversed as the unused bits octet would
sort in the opposite order. Any pair of IPAddressOrRange choices in
an extension MUST NOT overlap each other. Any contiguous address
prefixes or ranges MUST be combined into a single range or, whenever
possible, a single prefix.
2.2.3.7. Type IPAddressOrRange
The IPAddressOrRange type is a CHOICE of either an addressPrefix (an
IP prefix or address) or an addressRange (an IP address range)
element.
This specification requires that any range of addresses that can be
encoded as a prefix MUST be encoded using an IPAddress element (a BIT
STRING), and any range that cannot be encoded as a prefix MUST be
encoded using an IPAddressRange (a SEQUENCE containing two BIT
STRINGs). The following pseudo code illustrates how to select the
encoding of a given range of addresses.
LET N = the number of matching most-significant bits in the
lowest and highest addresses of the range
IF all the remaining bits in the lowest address are zero-bits
AND all the remaining bits in the highest address are one-bits
THEN the range MUST be encoded as an N-bit IPAddress
ELSE the range MUST be encoded as an IPAddressRange
2.2.3.8. Element addressPrefix and Type IPAddress
The addressPrefix element is an IPAddress type. The IPAddress type
defines a range of IP addresses in which the most-significant (left-
most) N bits of the address remain constant, while the remaining bits
(32 - N bits for IPv4, or 128 - N bits for IPv6) may be either zero
or one. For example, the IPv4 prefix 10.64/12 corresponds to the
addresses 10.64.0.0 to 10.79.255.255, while 10.64/11 corresponds to
10.64.0.0 to 10.95.255.255. The IPv6 prefix 2001:0:2/48 represents
addresses 2001:0:2:: to 2001:0:2:ffff:ffff:ffff:ffff:ffff.
An IP address prefix is encoded as a BIT STRING. The DER encoding of
a BIT STRING uses the initial octet of the string to specify how many
of the least-significant bits of the last subsequent octet are
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RFC 3779 X.509 Extensions for IP Addr and AS ID June 2004
unused. The DER encoding specifies that these unused bits MUST be
set to zero-bits.
Example:
128.0.0.0 = 1000 0000.0000 0000.0000 0000.0000 0000
to 143.255 255 255 = 1000 1111.1111 1111.1111 1111.1111 1111
bit string to encode = 1000
Type Len Unused Bits ...
Encoding = 0x03 0x02 0x04 0x80
2.2.3.9. Element addressRange and Type IPAddressRange
The addressRange element is of type IPAddressRange. The
IPAddressRange type consists of a SEQUENCE containing a minimum
(element min) and maximum (element max) IP address. Each IP address
is encoded as a BIT STRING. The semantic interpretation of the
minimum address in an IPAddressRange is that all the unspecified bits
(for the full length of the IP address) are zero-bits. The semantic
interpretation of the maximum address is that all the unspecified
bits are one-bits. The BIT STRING for the minimum address results
from removing all the least-significant zero-bits from the minimum
address. The BIT STRING for the maximum address results from
removing all the least-significant one-bits from the maximum address.
Example:
129.64.0.0 = 1000 0001.0100 0000.0000 0000.0000 0000
to 143.255.255.255 = 1000 1111.1111 1111.1111 1111.1111 1111
minimum bit string = 1000 0001.01
maximum bit string = 1000
Encoding = SEQUENCE {
Type Len Unused Bits ...
min 0x03 0x03 0x06 0x81 0x40
max 0x03 0x02 0x04 0x80
}
To simplify the comparison of IP address blocks when performing
certification path validation, a maximum IP address MUST contain at
least one bit whose value is 1, i.e., the subsequent octets may not
be omitted nor all zero.
2.3. IP Address Delegation Extension Certification Path Validation
Certification path validation of a certificate containing the IP
address delegation extension requires additional processing. As each
certificate in a path is validated, the IP addresses in the IP
address delegation extension of that certificate MUST be subsumed by
IP addresses in the IP address delegation extension in the issuer's
certificate. Validation MUST fail when this is not the case. A
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RFC 3779 X.509 Extensions for IP Addr and AS ID June 2004
certificate that is a trust anchor for certification path validation
of certificates containing the IP address delegation extension, as
well as all certificates along the path, MUST each contain the IP
address delegation extension. The initial set of allowed address
ranges is taken from the trust anchor certificate.
3. Autonomous System Identifier Delegation Extension
This extension conveys the allocation of autonomous system (AS)
identifiers to an entity by binding those AS identifiers to a public
key belonging to the entity.
3.1. Context
AS identifier delegation is currently managed by a hierarchy
nominally rooted at IANA, but managed by the RIRs. IANA allocates AS
identifiers to the RIRs, who in turn assign AS identifiers to
organizations who are end entities, i.e., will not be re-allocating
any of their AS identifiers to other organizations. The AS
identifier delegation extension is intended to enable verification of
the proper delegation of AS identifiers, i.e., of the authorization
of an entity to use these AS identifiers. Accordingly, it makes
sense to take advantage of the inherent authoritativeness of the
existing administrative framework for management of AS identifiers.
As described in Section 1 above, this will be achieved by issuing
certificates carrying the extension described in this section. An
example of one use of the information in this extension is an entity
using it to verify the authorization of an organization to manage the
AS identified by an AS identifier in the extension. The use of this
extension to represent assignment of AS identifiers is not intended
to alter the procedures by which AS identifiers are managed, or when
an AS should be used c.f., [RFC1930].
3.2. Specification3.2.1. OID
The OID for this extension is id-pe-autonomousSysIds.
id-pe-autonomousSysIds OBJECT IDENTIFIER ::= { id-pe 8 }
where [RFC3280] defines:
id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7) }
id-pe OBJECT IDENTIFIER ::= { id-pkix 1 }
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RFC 3779 X.509 Extensions for IP Addr and AS ID June 20043.2.2. Criticality
This extension SHOULD be CRITICAL. The intended use of this
extension is to connote a right-to-use for the AS identifiers in the
extension. A CA marks the extension as CRITICAL to convey the notion
that a relying party must understand the semantics of the extension
to make use of the certificate for the purpose it was issued. Newly
created applications that use certificates containing this extension
are expected to recognize the extension.
3.2.3. Syntax
id-pe-autonomousSysIds OBJECT IDENTIFIER ::= { id-pe 8 }
ASIdentifiers ::= SEQUENCE {
asnum [0] EXPLICIT ASIdentifierChoice OPTIONAL,
rdi [1] EXPLICIT ASIdentifierChoice OPTIONAL}
ASIdentifierChoice ::= CHOICE {
inherit NULL, -- inherit from issuer --
asIdsOrRanges SEQUENCE OF ASIdOrRange }
ASIdOrRange ::= CHOICE {
id ASId,
range ASRange }
ASRange ::= SEQUENCE {
min ASId,
max ASId }
ASId ::= INTEGER
3.2.3.1. Type ASIdentifiers
The ASIdentifiers type is a SEQUENCE containing one or more forms of
autonomous system identifiers -- AS numbers (in the asnum element) or
routing domain identifiers (in the rdi element). When the
ASIdentifiers type contains multiple forms of identifiers, the asnum
entry MUST precede the rdi entry. AS numbers are used by BGP, and
routing domain identifiers are specified in the IDRP [RFC1142].
3.2.3.2. Elements asnum, rdi, and Type ASIdentifierChoice
The asnum and rdi elements are both of type ASIdentifierChoice. The
ASIdentifierChoice type is a CHOICE of either the inherit or
asIdsOrRanges element.
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RFC 3779 X.509 Extensions for IP Addr and AS ID June 20043.2.3.3. Element inherit
If the ASIdentifierChoice choice contains the inherit element, then
the set of authorized AS identifiers is taken from the issuer's
certificate, or from the issuer's issuer's certificate, recursively,
until a certificate containing an ASIdentifierChoice containing an
asIdsOrRanges element is located. If no authorization is being
granted for a particular form of AS identifier, then there MUST NOT
be a corresponding asnum/rdi member in the ASIdentifiers sequence.
3.2.3.4. Element asIdsOrRanges
The asIdsOrRanges element is a SEQUENCE of ASIdOrRange types. Any
pair of items in the asIdsOrRanges SEQUENCE MUST NOT overlap. Any
contiguous series of AS identifiers MUST be combined into a single
range whenever possible. The AS identifiers in the asIdsOrRanges
element MUST be sorted by increasing numeric value.
3.2.3.5. Type ASIdOrRange
The ASIdOrRange type is a CHOICE of either a single integer (ASId) or
a single sequence (ASRange).
3.2.3.6. Element id
The id element has type ASId.
3.2.3.7. Element range
The range element has type ASRange.
3.2.3.8. Type ASRange
The ASRange type is a SEQUENCE consisting of a min and a max element,
and is used to specify a range of AS identifier values.
3.2.3.9. Elements min and max
The min and max elements have type ASId. The min element is used to
specify the value of the minimum AS identifier in the range, and the
max element specifies the value of the maximum AS identifier in the
range.
3.2.3.10. Type ASId
The ASId type is an INTEGER.
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RFC 3779 X.509 Extensions for IP Addr and AS ID June 20043.3. Autonomous System Identifier Delegation Extension Certification Path Validation
Certification path validation of a certificate containing the
autonomous system identifier delegation extension requires additional
processing. As each certificate in a path is validated, the AS
identifiers in the autonomous system identifier delegation extension
of that certificate MUST be subsumed by the AS identifiers in the
autonomous system identifier delegation extension in the issuer's
certificate. Validation MUST fail when this is not the case. A
certificate that is a trust anchor for certification path validation
of certificates containing the autonomous system identifier
delegation extension, as well as all certificates along the path,
MUST each contain the autonomous system identifier delegation
extension. The initial set of allowed AS identifiers is taken from
the trust anchor certificate.
4. Security Considerations
This specification describes two X.509 extensions. Since X.509
certificates are digitally signed, no additional integrity service is
necessary. Certificates with these extensions need not be kept
secret, and unrestricted and anonymous access to these certificates
has no security implications.
However, security factors outside the scope of this specification
will affect the assurance provided to certificate users. This
section highlights critical issues that should be considered by
implementors, administrators, and users.
These extensions represent authorization information, i.e., a right-
to-use for IP addresses or AS identifiers. They were developed to
support a secure version of BGP [S-BGP], but may be employed in other
contexts. In the secure BGP context, certificates containing these
extensions function as capabilities: the certificate asserts that the
holder of the private key (the Subject) is authorized to use the IP
addresses or AS identifiers represented in the extension(s). As a
result of this capability model, the Subject field is largely
irrelevant for security purposes, contrary to common PKI conventions.
5. Acknowledgments
The authors would like to acknowledge the contributions to this
specification by Charles Gardiner, Russ Housley, James Manger, and
Jim Schaad.
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RFC 3779 X.509 Extensions for IP Addr and AS ID June 2004
IPAddressOrRange
03 04 04 0a0020 addressPrefix 10.0.32/20
IPAddressOrRange
03 04 00 0a0040 addressPrefix 10.0.64/24
IPAddressOrRange
03 03 00 0a01 addressPrefix 10.1/16
IPAddressOrRange
30 0c addressRange {
03 04 04 0a0230 min 10.2.48.0
03 04 00 0a0240 max 10.2.64.255
} -- addressRange
IPAddressOrRange
03 03 00 0a03 addressPrefix 10.3/16
} -- addressesOrRanges
} -- IPAddressFamily
30 06 IPAddressFamily {
04 02 0002 addressFamily: IPv6
IPAddressChoice
05 00 inherit from issuer
} -- IPAddressFamily
} -- IPAddrBlocks
} -- extnValue
} -- Extension
This example illustrates how the prefixes and ranges are sorted.
+ Prefix 1 MUST precede prefix 2, even though the number of unused
bits (4) in prefix 1 is larger than the number of unused bits (0)
in prefix 2.
+ Prefix 2 MUST precede prefix 3 even though the number of octets
(4) in the BIT STRING encoding of prefix 2 is larger than the
number of octets (3) in the BIT STRING encoding of prefix 3.
+ Prefixes 4 and 5 are adjacent (representing the range of addresses
from 10.2.48.0 to 10.2.64.255), so MUST be combined into a range
(since the range cannot be encoded by a single prefix).
+ Note that the six trailing zero bits in the max element of the
range are significant to the semantic interpretation of the value
(as all unused bits are interpreted to be 1's, not 0's). The four
trailing zero bits in the min element are not significant and MUST
be removed (thus the (4) unused bits in the encoding of the min
element). (DER encoding requires that any unused bits in the last
subsequent octet MUST be set to zero.)
Lynn, et al. Standards Track [Page 19]

RFC 3779 X.509 Extensions for IP Addr and AS ID June 2004
authorization from an Internet service provider, who in turn gets its
authorization from a RIR. Note that AS identifiers might be sub-
allocated in the future, so the mechanisms used should not rely upon
a three level hierarchy.
In section 1 of RFC 3281, two reasons are given for why the use of
ACs might be preferable to the use of public key certificates (PKCs)
with extensions that convey the authorization information:
"Authorization information may be placed in a PKC extension or
placed in a separate attribute certificate (AC). The placement of
authorization information in PKCs is usually undesirable for two
reasons. First, authorization information often does not have the
same lifetime as the binding of the identity and the public key.
When authorization information is placed in a PKC extension, the
general result is the shortening of the PKC useful lifetime.
Second, the PKC issuer is not usually authoritative for the
authorization information. This results in additional steps for
the PKC issuer to obtain authorization information from the
authoritative source."
"For these reasons, it is often better to separate authorization
information from the PKC. Yet, authorization information also
needs to be bound to an identity. An AC provides this binding; it
is simply a digitally signed (or certified) identity and set of
attributes."
In the case of the IP address and AS identifier authorizations, these
reasons do not apply. First, the public key certificates are issued
exclusively for authorization, so the certificate lifetime
corresponds exactly to the authorization lifetime, which is often
tied to a contractual relationship between the issuer and entity
receiving the authorization. The Subject and Issuer names are only
used for chaining during certification path validation, and the names
need not correspond to any physical entity. The Subject name in the
PKCs may actually be randomly assigned by the issuing CA, allowing
the resource holder limited anonymity. Second, the certificate
hierarchy is constructed so that the certificate issuer is
authoritative for the authorization information.
Thus the two points in the first cited paragraph above are not true
in the case of AS number and IP address block allocations. The point
of the second cited paragraph is also not applicable as the resources
are not being bound to an identity but to the holder of the private
key corresponding to the public key in the PKC.
Lynn, et al. Standards Track [Page 22]

RFC 3779 X.509 Extensions for IP Addr and AS ID June 2004RFC 3281 specifies several requirements that a conformant Attribute
Certificate must meet. In relation to S-BGP, the more-significant
requirements are:
1 from section 1: "this specification does NOT RECOMMEND the use of
AC chains. Other (future) specifications may address the use of
AC chains."
Allocation from IANA to RIRs to ISPs to DSPs and assignment to end
organizations would require the use of chains, at least for IP
address blocks. A description of how the superior's AC should be
located and how it should be processed would have to be provided.
Readers of this document are encouraged to propose ways the
chaining might be avoided.
2 from section 4.2.9: "section 4.3 defines the extensions that MAY
be used with this profile, and whether or not they may be marked
critical. If any other critical extension is used, the AC does
not conform to this profile. However, if any other non-critical
extension is used, the AC does conform to this profile."
This means that the delegation extensions defined in this
specification, which are critical, could not be simply placed into
an AC. They could be used if not marked critical, but the
intended use requires that the extensions be critical so that the
certificates containing them cannot be used as identity
certificates by an unsuspecting application.
3 from section 4.5: "an AC issuer, MUST NOT also be a PKC issuer.
That is, an AC issuer cannot be a CA as well."
This means that for each AC issuer there would need to be a
separate CA to issue the PKC that contains the public key of the
AC holder. The AC issuer cannot issue the PKC of the holder, and
the PKC issuer cannot sign the AC. Thus, each entity in the PKI
would need to operate an AC issuer in addition to its CA. There
would be twice as many certificate issuers and CRLs to process to
support Attribute certificates than are needed if PKCs are used.
The possibility of mis-alignment also arises when there are two
issuers issuing certificates for a single purpose.
The AC model of RFC 3281 implies that the AC holder presents the
AC to the AC verifier when the holder wants to substantiate an
attribute or authorization. The intended usage for the extensions
defined herein does not have a direct interaction between an AC
verifier (a NOC) and the AC issuers (all RIRs and NOCs). Given a
Lynn, et al. Standards Track [Page 23]

RFC 3779 X.509 Extensions for IP Addr and AS ID June 2004
signature on a claimed right-to-use object, the "AC verifier" can
locate the AC holder's PKC, but there is no direct way to locate
the Subject's AC(s).
4 from section 5: "4. The AC issuer MUST be directly trusted as an
AC issuer (by configuration or otherwise)."
This is not true in the case of a right-to-use for an IP address
block, which is allocated through a hierarchy. Certification path
validation of the AC will require chaining up through the
delegation hierarchy. Having to configure each relying party
(NOC) to "trust" every other NOC does not scale, and such "trust"
has resulted in failures that the proposed security mechanisms are
designed to prevent. A single PKI with a trusted root is used,
not thousands of individually trusted per-ISP AC issuers.
The amount of work that would be required to properly validate an
AC is larger than for the mechanism that places the certificate
extensions defined in this document in the PKCs. There would be
twice as many certificates to be validated, in addition to the
ACs. There could be a considerable increase in the management
burden required to support ACs.
References
Normative References
[IANA-AFI] http://www.iana.org/assignments/address-family-numbers.
[IANA-SAFI] http://www.iana.org/assignments/safi-namespace.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Level", BCP 14, RFC 2119, March 1997.
[RFC3280] Housley, R., Polk, W., Ford, W. and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[X.690] ITU-T Recommendation X.690 (1997) | ISO/IEC 8825-1:1998,
"Information Technology - ASN.1 Encoding Rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)".
Lynn, et al. Standards Track [Page 24]

RFC 3779 X.509 Extensions for IP Addr and AS ID June 2004
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Lynn, et al. Standards Track [Page 27]